The present invention relates generally to an optical unit and an exposure apparatus having the same, and more particularly, to an optical unit that adjusts the shape of an optical element and an exposure apparatus having the same. This invention is suitable, for example, for an optical unit that adjusts a surface shape of a mirror to correct a wavefront aberration of an optical system including the mirror, as well as an exposure apparatus having such an optical system.
In fabricating a semiconductor device, and the like, by using a photolithography process, a projection exposure apparatus has so far been used that transfers a pattern of a reticle (mask) to a plate. Since such an exposure apparatus is required to precisely transfer a reticle pattern onto a plate, a projection optical system needs to be used that is excellent in imaging performance and well controls aberrations. Due to the recent demand for finer semiconductor devices, a pattern to be transferred has become very susceptible to aberrations of an optical system. For this reason, it is demanded to highly precisely correct wavefront aberration of a projection optical system.
There are two conventional aberration correction approaches—one that adjusts an optical element's surface shape (e.g., see PCT Japanese Publication No. 2003-519404) and the other that changes a position (or posture) of an optical element while maintaining the surface shape of the optical element (e.g., see Japanese Patent Application, Publication No. 2004-064076). PCT Japanese Publication No. 2003-519404 discloses a two-tiered mirror deforming means having actuators connected to the back of a mirror, which correct high-order aberrations, and actuators connected to the actuators via an intermediary deformation plate, which correct low-order aberrations. Japanese Patent Application, Publication No. 2004-064076, discloses a mechanism that uses at least three force actuators to move abreast to an optical axial direction of a mirror or rotate to a direction perpendicular to the optical axis, etc.
PCT Japanese Publication No. 2003-519404 arranges two actuators via an intermediary deformation plate, complicates controls, and results in a large size, due to the two-tier method. Moreover, any asymmetrical aberration causes a moment on the mirror, displacing the mirror's position, but PCT Japanese Publication No. 2003-519404 is silent as to the orientation control. In Japanese Patent Application, Publication No. 2004-064076, actuators act directly on the back surface of the mirror, and the surface shape of a mirror may locally deform during control of the mirror's posture. Any actuator-related part fixed to the back surface of the mirror would deform the mirror due to the weight of the part, but Japanese Patent Application, Publication No. 2004-064076, is silent as to the own-weight compensation.